Cross polarization experiments in NMR provide a vehicle for enhancing the resonance signal for nuclei which exhibit a low value of the gyromagnetic ratio .gamma..sub.s through transfer of magnetization from a resonant nuclei exhibiting a high gyromagnetic ratio .gamma..sub.i in relation to the nuclei of .gamma..sub.s. This transfer occurs when the separate rotating frame precession frequencies for the two nuclear species are caused to coincide yielding the Hartman-Hahn condition. EQU .gamma..sub.I B.sub.1.sup.(I) =.gamma..sub.s B.sub.1.sup.(s)
Where the subscripts and superscripts label respective nuclei of low (S) and high (I) gyromagnetic ratio such as C.sup.13 and protons, and the corresponding rf magnetic fields.
For high resolution NMR it is most often desirable to mechanically rotate the sample about a selected axis. This mechanical rotation is ordinarily accomplished with a pneumatic apparatus for the purpose of averaging inhomogeneities of the field and, for solids, inhomogeneities of the sample structure. Rotation rates of the order of 10.sup.3 -10.sup.4 Hertz are typical. One result of a mechanical rotation is the imposition of spinning side-bands on the NMR excitation and the NMR signal with the result that stringent conditions are imposed on the rf magnetic field amplitudes observed at the sample in a cross polarization experiment.
In one technique of prior art the modulation arising from the spinning operation is removed by arranging for the spinner to be stationary during a cross polarization evolution period, t.sub.1. At the conclusion of the interval t.sub.1, the transverse magnetization is protected by application of an appropriately phased 90.degree. pulse to preserve the magnetic properties of the system for a brief period while the spinner is brought up to speed, after which the experiment is completed with the rf fields applied to the respective spin subsystems. In this approach the measurement requires great expenditure of time to allow for the rotor to return to a stable rotation at the desired rate. During this return to the stable rotation condition some loss of magnetization will be experienced (as a function T.sub.1) thereby degrading the signal to noise ratio. An example of this prior art approach will be found in J. Mag. Res., Vol. 79, pp. 299-306 (1985).
It is also known to synchronize rf pulses with the sample rotor at multiple angular intervals of sample rotation following a period of cross polarization for the purpose of adapting the width of a so-called powder pattern (continuous spectrum arising from random oriented crystallites) to a desired frequency interval so as to avoid overlap of powder patterns. The technique is discussed by Yarin-Agaev et al., J. Mag. Res., G. 47, pp. 51-60 (1982).
In the present invention the efficiency of polarization transfer and certain other benefits are found to result from the use of rotor synchronized pulses employed to establish the Hartman-Hahn condition in an improved manner. An important example of auxiliary benefit is that with the present invention the integrated resonant signal acquires greater independence of rotor speed and Hartman-Hahn efficiency, thereby providing a more reliable quantitative indicia of the density of specific nuclei. This consequence permits spinning at high rates, a desirable goal at the high magnetic fields characterizing the subject measurements. Further, the practice of this invention increases tolerance of mechanical instability which may accompany the highest spinning rates. Thus, there is not required the complexity of maintaining an exacting spin rate stabilization servo-loop.
The present invention is directed to removal or reduction of the spinning modulation imposed on a cross polarization condition due to sample spinning. It is accomplished by applying a rotor synchronous modulation to either, or both rf fields employed to produce the cross polarization condition.
The rotor-synchronous modulation may be directed to the rf amplitude or the phase. This modulation may be derived from the rotor by any of a number of well known arrangements.